[Ifeffit] Heavy Scatterer Resonance

[Matt Newville]

Pieter,

Please use the ifeffit mailing list for questions about XAFS Analysis.  I
am CCing this reply to that list.

On Thu, Jun 2, 2016 at 2:35 AM, Pieter Tack <Pieter.Tack at ugent.be> wrote:

> Dear Dr. Newville,
>
>
> During the analyses of some metallic Au EXAFS I noticed the Au-Au single
> scatter first shell is characterised by a split peak in the chi(R)
> spectrum. I was wondering what caused this, as I would expect a single
> scatter path resulting in only one scatter distance and thus one peak in
> the chi(R). However, I could not find much on the matter except for a quote
> of you stating it is "due to the resonance in the scattering for heavy
> scatterers" (
> http://permalink.gmane.org/gmane.science.physics.ifeffit/2352 )
>

In short:  XAFS chi(R) is *NOT* a pair distribution function, due mostly to
the complex electron scattering that alter the amplitude and shift the
phase of potoelectron in addition to the simplistic "backscatter from a
point source atom".   The most obvious effect of this is a ~0.5 Ang shift
to lower R from the value you might expect if chi(R) was a pair
distribution function.   Other effects include broader and asymmetric
peaks,  and the resonance effect such as you notice for heavy elements
(easily noticeable for Z>40 or so, but unmistakable for Z > 60  -- that Z
is for the *scattering atom*).

At certain energies (or for a narrow range of energies), the photo-electron
has very low scattering probability from a heavy element, and the phase
jumps dramatically, as if the electron was able to tunnel through or hop
over the potential well of the scattering atom.
This is usually (and I think properly) called a Ramsauer-Townsend resonance.

There are many papers, review articles, and slides from talks available
that describe this in more detail.  Google "ramsauer townsend effect exafs"
for detailed articles.

> I don't really understand what this means, which physical process causes
> this. Could you perhaps provide me with additional info on this matter or
> direct me to some papers that explain this?
>
>
> Thanks in advance!
>
> Best regards
>
> Pieter Tack
>
>
> ---
> X-Ray Micro-Spectroscopy and Imaging Group
> Department of Analytical Chemistry
> Ghent University
> Krijgslaan 281 S12
> B-9000 Ghent
> Belgium
> Phone: +32 (0)9 264 4723
> Fax: +32 (0)9 264 49 60
>



--Matt
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